In this invention, each term means the following.
[Explanation of terms]
Period
In general, the term "period" inherently means a dimension of time. In this invention, however, it also means a dimension of length in a path through which a predetermined light passes inside an optical crystal.
Frequency
While the light is generally expressed by "wavelength", it is expressed by "frequency" in this invention. The "wavelength" corresponding to this "frequency" varies depending on the material of transmission path, as in the case of radio waves.
Optical Parametric Oscillation
This means a phenomenon wherein two frequencies .omega..sub.s and .omega..sub.i are generated by excitation of quadratic polarization with a light wave having a frequency .omega..sub.p. The relation of these frequencies is as expressed by the formula: EQU .omega..sub.p =.omega..sub.s +.omega..sub.i.
Domain
A region wherein polarization in ferroelectrics occurs in the same direction. The references relevant to this invention include the following.
[References]
Reference 1
P. A. Franken, A. E. Hill, C. W. Peters, G. Weinreich, "Generation of optical harmonics", Phys. Rev. Lett., 7, 118/1961.
Reference 2
J. A. Armstrong, N. Bleombergen, J. Ducuing and P. S. Pershen, "Interaction between light waves in a nonlinear dielectric", Phys. Rev., 127, No. 6, 1918/1962.
Reference 3
Eikai Cho, Hiromasa Ito and Fumio Inaba, 49.sup.th Applied Physics Convention, "Experiment of nonlinear light waveguide having domain inversion structure", 7a-ZD-9/1988.
Reference 4
E. J. Lim, M. M. Fejer, R. L. Byer, and W. J. Kozlovsky, "Blue light generation by frequency doubling in periodically poled lithium niobate channel waveguide", Electron. Lett., 25, 731/1989.
Reference 5
J. Webjorn, F. Laurell, and G. Arvidsson, "Blue light generated by frequency doubling of laser diode light in a lithium niobate channel waveguide", IEEE Photon. Tech. Lett., 1, 316/1989.
Reference 6
M. Yamada, N. Nada, M. Saitoh, and K Watanabe, "First-order quasi-phase matched LiNbO.sub.3 waveguide periodically poled by applying an external field for efficient blue second-harmonic generation", Appl. Phys. Lett., 62, 435/1993).
Reference 7
Y. Yamamoto, S. Yamaguchi, K. Suzuki, and N. Yamada, "Second-harmonic generation in a waveguide with domain-inverted regions like periodic lens sequence on z-face KTiOPO.sub.4 crystal", Appl. phys. Lett., 65, 938/1994.
Reference 8
Toshiaki Saihara, Masatoshi Fujimura and Hiroshi Nishihara, "Waveguide type SHG element by quasi-phase matching", Journal of Electron Information Communication Convention, 76,597/1993.
Reference 9
S. Miyazawa, "Ferroelectric domain inversion in Ti-diffused LiNbO.sub.3 optical waveguide", J. Appl. Phys., 50, 4599/1979.
Reference 10
H. Ito, C. Takyu, and H. Inaba, "Fabrication of periodic domain grating in LiNbO.sub.3 by electron beam writing for the application of nonlinear optical processes", Electron. Lett., 27, 1221/1991.
Reference 11
Motoki Ohashi, Choichi Takyu and Koichi Taniguchi, "Studies of periodic domain inversion structure of ferroelectric nonlinear optical crystal by electron beam writing", Journal of Electron Information Communication Convention Papers, C-I, J77-C-I, 383/1994.
Reference 12
S. Kurimura, M. Miura, and I. Sawaki, "New method of 20 mm-deep and 3.6 mm-periodic domain inversion for lst-order quasi-phase matching SHG in LiTaO.sub.3 waveguides", Conf. on Lasers and Electro-Optics, CPD5/1992.
Reference 13
Manabu Sato, Motoki Ohashi, Abedin Kaji Sarwar, Choichi Takyu and Masahiro Ito, "Bulk LiTaO.sub.3 Domain Inversion Lattice by Electric Field Application Method", Journal of Electron Information Communication Convention Papers, C-I, 366, J78-C-I, August/1995.
Reference 14
K S. Abedin, M. Sato, H. Ito, T. Maeda, K. Shimamura, and T. Fukuda, "Ordinary and extraordinary continuous wave lasing at 1.092 .mu.m and 1.082 .mu.m in bulk Nd: LiTaO.sub.3 crystal", J. Appl. Phys., 78,691, July/1995.
Reference 15
A. YARIV et al., OPTICAL WAVES IN CRYSTALS, pp.512-515, Table 12.2, WILEY-INTERSCIENCE, 1983.
Reference 16
Electric Communication Handbook, Ed. 25, First Division, 3.multidot.2, "Q switch", Ohm Corp., March, 1979.
[Developmental history of optical device]
Subsequent to the invention of the laser, a technique for generating a second harmonic at 347 nm by ruby laser beam (694 nm) irradiation of rock crystal was disclosed in Reference 1.
Since then, nonlinear optics have been remarkably developed and have become an object of scientific studies, as well as a practical device technique in significant progress.
The development of light wave technique, as in the case of the development of radio wave technique, requires a light source having any wavelength, namely, a light source having any frequency. However, the wavelength of oscillation by laser is principally a specific wavelength determined by the material having a laser activity. Therefore, generation of a light having an optional wavelength, which cannot be obtained directly using a laser, is desired. As a technique for this end, a technique using a material having nonlinear optical character, namely, a nonlinear optical material, has been noticeably developed in recent years.
For an efficient generation of coherent light waves having different wavelengths by the use of a nonlinear optical material, namely, a material having nonlinear optical effect (to be referred to as nonlinear optical character in this invention), it is necessary to realize matching between the velocity at which a new wavelength component, namely, a frequency component, created by mixing plural light waves is polarized, and the transmission velocity of the light wave emitted by the polarization. The matching of the velocities is called phase matching and is generally achieved by the use of a crystal having a birefingence property.
There is a method to achieve phase matching without relying solely on the birefringence property, that is, a method free of limitations in conventional phase matching, and such phase matching method is called a quasi-phase matching. According to this method, the maximum value of a nonlinear optical coefficient component, namely, a nonlinear optical tensor component, can be utilized. The tensor component is disclosed in Reference 15. While the principle proposition of quasi-phase matching appears in Reference 2, due to the difficulty in rotating the optic axis z of a crystal (hereinafter "optical axis z") periodically and precisely by 180.degree. on a .mu.m order, no specific device has been realized.
The inventor of this invention has disclosed, in Reference 3, a method for quasi-phase matching by a structure capable of periodic domain inversion by diffusion of impurities at periodic intervals on the surface of a material having ferroelectricity due to LiNbO.sub.3 (lithium niobate), namely, a ferroelectric crystal. This periodic domain inversion structure can be manufactured by planar processing alone which is similar to that applied to a semiconductor device, and this structure achieves generation of second harmonic of a near infrared ray. Nearly at the time when the above-mentioned Reference 3 reported the technique, similar techniques were disclosed in Reference 4 and Reference 5, and improvements of these techniques were disclosed in References 6-8.
Moreover, the inventor of this invention has disclosed, in References 10, 11 and 13, a technique affording, with regard to a ferroelectric crystal, domain control with precision on a micron order, of a domain inversion structure over the entire substrate, namely, the entire crystal, by an electric means, which domain inversion structure having been made to permit quasi-phase matching by utilizing a nonlinear optical coefficient in a more ensured manner. Specifically, this technique has realized a second harmonic generation at an efficiency near a theoretical value, by forming a periodic domain inversion part of not more than 10 .mu.m in a z board of LiTaO.sub.3 (lithium tantalate) having a thickness of 500 .mu.m, namely, a crystal board having the optical axis z direction set as in FIG. 19.
In addition, a structure generating millimeter wave to submillimeter wave having frequency .omega..sub.3, which is the difference in light wave between two different frequencies .omega..sub.1 and .omega..sub.2, via quasi-phase matching, as shown in FIG. 18, has been disclosed in Japanese Patent Unexamined Publication No. 6-110095. The "nonlinear optical crystal wherein nonlinear optical coefficient is periodically inverted" in the above-mentioned Japanese Patent Unexamined Publication No. 6-110095 and the periodic domain inversion part in this invention have substantially the same construction.
[quasi-phase matching]
The principle of the generation of harmonic by quasi-phase matching is explained in the following.
In general terms, nonlinear interactions between different wavelengths require phase matching to preserve momentum and to preserve energy. When such a requirement is not fulfilled, a nonlinear polarization wave induced in a substance by incident light wave and the light wave emitted by this nonlinear polarization wave interfere with each other to cancel each other, thus resulting in a failure to achieve effective conversion of frequencies.
In a second harmonic generation (hereinafter SHG), as shown in FIG. 10(c), harmonic output includes constant maximum level and minimum level repeats at an extremely weak intensity, in every interference distance calculated by the following formula, namely, length of coherence, Lc. ##EQU1##
wherein .lambda. is a wavelength, n is a refractive index, an inferior subscript F is a fundamental wave, and SH means harmonic component. Therefore, once the +/- symbols of the polarization wave generated every coherence length, Lc, can be alternately inverted by some means, the harmonic output can be effectively overlapped over the entire part subjected to the alternate inversion. In other words, by setting the period of domain inversion, namely, the domain period T, to twice the length of coherence, Lc, namely, 2Lc, the diffusion inside the nonlinear optical crystal can be cancelled to simulate phase matching, as shown in FIG. 10, part (a).
The stimulatory phase matching in this way is called quasi-phase matching (hereinafter to be referred to as QPM. A nonlinear optical material having a great tensor component but optically isotropic, or incapable of phase matching due to too great a dispersion, can be subjected to phase matching by this quasi-phase matching, thereby enabling utilization of such nonlinear optical material for SHG.
When the domain period T is set to thrice the length of coherence, Lc, namely, 3Lc, the outcome will be as shown in FIG. 10, part (b). The conditions to be fulfilled to perform QPM are generally expressed by the following formula: ##EQU2##
wherein m is a positive integer of 1, 2, 3 . . . and is an order of domain period T. In FIG. 10, (a) is the case where T=Lc, m=1; (b) is the case where T=3Lc, m=3; and (c) is the case where such domain period is not set, namely, phase mismatch.
When using a material dimensionally controlled to effect inversion as exemplified by periodic inversion of nonlinear optical coefficient d into +d and -d, the dimensional distribution of this nonlinear coefficient is subjected to Fourier expansion to determine the effective nonlinear optical coefficient.
In this case, when the ratio of the width of the part wherein z axis is substantially inverted about the z axis of the crystal substrate, to domain period T, namely, the ratio of the inverted width t to the domain period T in FIG. 14, which is called a duty ratio, is expressed by .xi. (0.ltoreq..xi..ltoreq.1), the maximum efficiency of QPM can be achieved when m=1 and .xi.=0.5. At a higher order action, the highest efficiency can be achieved when m=2 and .xi.=0.25. Accordingly, for a highly efficient quasi-phase matching, the control of the duty ratio .xi. is essential.
On the other hand, a nonlinear optical material is requested to have a broad pass band and a large nonlinear optical coefficient d, since it is used in the wavelength band where it is transmitted, namely, transmission frequency band. The relationship between the cut-off point of pass band at short wavelength side, i.e., high frequency side, of each optical material, and a relative property index d.sup.2 /n.sup.3 is as shown in FIG. 11.
In this quasi-phase matching, therefore, an optical material having a greater nonlinear optical coefficient d can be used under the optimal operative conditions free of limitations imposed on conventional phase matching. As shown in FIG. 11, LiNbO.sub.3 (QPM) and LiTaO.sub.3 (QPM) by QPM show properties superior to that of LiNbO.sub.3 and LiTaO.sub.3 without QPM.
A material having such a nonlinear optical character is required to be highly efficient. In addition, for a greater output to be achieved, the input should be also great, and such great input may destroy the nonlinear optical material. Thus, this material is suitable for a nonlinear optical interaction at low and medium output levels.
The relation between a transmission wavelength range of each typical optical crystal suitable for QPM and a nonlinear optical coefficient, namely, tensor components d.sub.33, d.sub.31 is shown in FIG. 17, and the use of tensor component d.sub.33 results in better QPM than the use of other tensor components such as tensor component d.sub.31.
However, since the tensor component d.sub.33 is usable when all electric fields to be interacted are parallel to the optical axis z of an optical crystal, it cannot be used at all in an interaction using conventional birefringence.
In other words, according to the QPM, LiNbO.sub.3 crystal, for example, permits effective use of the value of -40 pm/V of tensor component d.sub.33, which is about 7 times greater than that of the conventional tensor component d.sub.31, which is -6 pm/V. Thus, the QPM enables the use of a nonlinear optical coefficient which is about 7 times greater than the conventional one. As shown in FIG. 17, it is one of the materials having the greatest nonlinear character in the wavelength range of not more than wavelength 0.4 .mu.m.
[periodic domain inversion structure]
For periodic inversion of poling direction in a crystal as shown by the symbols of the polarized waves in FIG. 18, the +/- symbols of nonlinear optical coefficients need only be reversed. Therefore, a method for reversing the optical axis, namely, z axis, inside the optical crystal can be used. Such inversion of the z axis is called domain inversion in this invention. When the ferroelectric is a crystal of LiNbO.sub.3 or LiTaO.sub.3, a structure wherein the ferroelectric domains are alternately inverted by 180.degree. at certain periods, namely, a periodic domain inversion part 15, need only be formed.
Reference 9 and others disclose that, particularly in +z plane, namely, a plane orthogonal to the optical axis z of a crystal and disposed on the plus direction side of the z axis, the domain inversion occurs by internal factors such as impurities and distortion, and external factors such as heat and electric field. Such inversion is considered to be mainly caused by lowered Curie temperature in the part where impurities are dispersed. Therefore, when a domain inversion structure is manufactured by surface processing based on the dispersion of the impurities, namely, by processing from the surface of the crystal, while positively utilizing this major cause, the degree of freedom in structural development of the device can be markedly increased as compared to the manufacture method employed to form domain inversion structure having an optional shape, along with conventional crystal pulling.
Reference 3--Reference 5 disclose a technique of such domain control of a nonlinear optical material by processing from the surface. However, the domain inversion based on the diffusion of Ti (titanium) carried out in early stages was associated with disadvantages in that, since changes in refractive index were always observed, it was susceptible to diffraction and dispersion, and that treatment at high temperatures was necessary.
In order to overcome this inconvenience, the inventor of this invention performed periodic inversion of domain only with electron beam and electric field at room temperature, in an attempt to realize a periodic domain inversion structure without variations in refractive index, and disclosed, in Reference 10, a method for causing domain inversion by electron beam irradiation alone without application of heat or electric field.
This method comprises deposition of a metal such as chromium on the +z plane of LiNbO.sub.3 substrate and exposure, to electron beam, of the part on the -z plane without deposition, namely, the plane opposite from the +z plane, where domain inversion is desired, whereby a desired pattern is drawn thereon to give a periodic domain inversion structure.
The electron beam is hit using a modified scanning electronmicroscope. For example, when a domain inversion at a period of 7.5 .mu.m is desired on a z board (500 .mu.m thick) as a substrate, entire process scanning is performed at accelerating voltage of 25 kV, a dose of 2.times.10.sup.9 electrons/sec at zero DC bias at room temperature. According to the conventional method, a DC voltage needs to be applied to the both sides of a z board having an elevated temperature. This method obliterates such operation.
The surface of LiNbO.sub.3 having a domain inversion structure formed by periodic domain inversion by the above-mentioned electron beam irradiation was etched and observed by an optical microscopy. As a result, as shown in [-z plane microscopic photograph] and [+z plane microscopic photograph] of FIG. 12, domain inversion by electron beam irradiation was successively performed not only on the -z plane on the electron beam irradiation side but also to the +z plane on the rear side, and y plane, namely, a plane orthogonal with the mechanical axis y of the crystal, after cutting, polishing, etching and similar observation, showed extremely regular formation of main inversion layers from -z plane to +z plane. Here, the relation of electron beam irradiation and each plane of the optical crystal is as shown in FIG. 19.
The process of domain inversion formation is considered to be such that accelerated electron beam advances to the depth of only 1-2 .mu.m from the crystal surface to charge only locally, and at the vicinity of the surface, binding of atoms is loosened due to the injection impact of electrons to cause easy movement of the atoms, which leads to dislocation of Li (lithium) ions now easily mobile in local electric field and inversion of spontaneous polarization, which inversion once caused at a certain part is repeated in the z axis direction of the created electric field to ultimately reach +z plane on the rear surface.
However, the domain inversion by the above-mentioned electron beam tends to be associated with the difficulty in forming a successive dean shape. This is caused by a slight imbalance between the electric charge accumulated on the insulated part and the electric charge from spontaneous inversion of the substrate, wherein the charges are not completely offset but remain. This inconvenience is disclosed in Reference 11.
On the other hand, a potential domain control by direct electric field is expected and a method of domain inversion by direct application of electric field to a ferroelectric via stripe-patterned electrodes has been studied. Reference 6 discloses domain inversion of LiNbO.sub.3 by the application of pulse electric field at around room temperature, and Reference 12 discloses domain inversion of LiTaO.sub.3 by the application of electric field. In these methods involving application of electric fields, the voltage to be applied should be strictly controlled, since the voltage is near dielectric breakdown voltage of crystal.
Thus, the inventor of this invention has disclosed, in Reference 13, a method for forming a bulk domain inversion lattice wherein periodic domain inversion is performed not only on the surface of a nonlinear optical LiTaO.sub.3 crystal but also in mostly the entire thickness direction of the optical crystal by the application of electric field.
According to this method, for example, a LiTaO.sub.3 z board (500 .mu.m thick) as a substrate, which is a plate crystal having a surface plane and a bottom plane both orthogonal to optical axis z, is used Using the plane on the plus side of optical axis z as +z plane and that on the minus side as -z plane, a certain pattern is formed on the +z plane, namely, Al (aluminum) electrode having a stripe pattern formed by periodic domain inversion is deposited on the +z plane in a certain pattern, and Al electrode is uniformly deposited on the entirety of -z plane. The substrate is set in a vacuum chamber, and a direct voltage is applied between the electrodes on both sides. The stripe patterned electrode has a length of 4 mm, and three periods of 7.5 .mu.m, 7.8 .mu.m and 8.1 .mu.m are formed simultaneously at different positions on the same substrate.
The time-course changes in the applied voltage and inversion current were, as shown in FIG. 13, that the applied voltage was increased at ca. 2.5 kV/10 sec; domain inversion current was 10.4 kV, which means that it began to flow from the vicinity of the voltage corresponding to 20.8 kv/mm; when the applied voltage was maintained at a constant value of ca. 10.5 kV, the inversion current reached the maximum value of ca. 750 nA after a certain period of time, at which level the domain inversion occurred in the patterned part; and about 1 minute later, the current stopped automatically. The applied voltage from which inversion current began to flow matched with the anti-electric field voltage of LiTaO.sub.3 at room temperature, namely, voltage before the occurrence of voltage breakage.
With regard to the injection charge for the above-mentioned domain inversion, since the charge during spontaneous polarization on the surface of LiTaO.sub.3 substrate is generally electrically neutralized with positive ion or electron, in the case of domain inversion by the application of electric field, the total amount of injected charge doubles during spontaneous polarization, upon injection of positive charge from the upper electrode, as in the case of domain inversion by electron beam irradiation, due to the neutralization of the surface charge and generation of electric field by domain inversion.
In the embodiment shown in FIG. 13, the injected charge measured was 27.6 .mu.C which is almost the same as 28.0 .mu.C by calculation. The injected charge corresponds to the area of the pattern of the patterned electrode. Therefore, it can be used as a parameter of domain control.
[Generation of second harmonic by quasi-phase matching]
In the generation of a second harmonic by a LiTaO.sub.3 crystal, the relationship between the wavelength of the fundamental wave when QPM order is first order (m=1) and second order (m=2), and the period of domain inversion as calculated by the formula (2) is expressed by the solid line in FIG. 15.
The evaluation test of LiTaO.sub.3 which underwent periodic domain inversion by the application of direct voltage by the above-mentioned patterned electrode was run by input-processing the output of the second harmonic by a computer, while continuously sweeping the oscillation wavelength of Ti:Al.sub.2 O.sub.3 (titanium sapphire) laser.
The relationship between the generation intensity of second harmonic at different domain periods T, namely, 7.5 .mu.m, 7.8 .mu.m and 8.1 .mu.m and the wavelength .lambda..sub.F of the fundamental wave at the .circleincircle. wherein order m=2 in FIG. 15, is shown in FIG. 16.
The double-dotted line in FIG. 15 represents the value at wavelength 1064 nm of Nd:YAG (neodymium.yttrium.aluminum.garnet) laser widely used as a solid-state laser, which indicates quasi-phase matching at a domain period of 7.8 .mu.m.
In addition, the width of the synchronization spectrum of fundamental wave at the generation intensity of second harmonic relative to each domain period T, namely, half-value breadth, was ca. 0.81 nm and almost the same for the three curves in FIG. 16.
Since the theoretical value of this width is 0.74 nm, the periodic domain inversion parts are considered to have been almost uniformly prepared, though with standard error within the range of experimental manufacture error. In addition, the experiment on the .circleincircle. point wherein m=1 in FIG. 15 was for the confirmation by the output of optical parametric oscillation. In this case again, the experimental results match well with the calculation results.
Since the light wave is subject to diffraction during transmission, it is essential for a the most efficient generation of second harmonic to collect light wave of the wavelength corresponding to the length of beam path in an optical device, namely, the length of the device. Under the conditions wherein conforcal length of the gauss beam and the above-mentioned device length were synchronized, the conversion efficiency .eta..sub.b is calculated from the following formula. ##EQU3##
wherein P.sub.SH is output of second harmonic, P.sub.F is input of fundamental wave, L is device length mentioned above, c is light speed, .mu..sub.0 is permeability of vacuum, .epsilon..sub.0 is permittivity of vacuum, .omega..sub.F is angular frequency of fundamental wave and d.sub.eff is effective second order nonlinear optical coefficient.
The reason for using the second order nonlinear optical coefficient here is that the nonlinear optical coefficient includes all nonlinear optical coefficients, such as second order, third order, fourth order and so on, but the generation of second harmonic by quasi-phase matching requires the use of only second order nonlinear optical coefficient.
In QPM wherein m=2, P.sub.F was 32 mW, L was 4 mm and P.sub.SH was 1.9 .mu.W.
From these results, it is known from formula (3) that the experimental value of .eta..sub.b was 0.46%/W cm. Since the duty ratio .xi. of the domain inversion structure prepared was ca. 0.63 by measurement, .eta..sub.b was 0.48%/W cm, similarly from formula (3). This means that the experimental value and the theoretical value are almost the same. However, by setting the duty ratio .xi. to an optimal value of 0.5, the value of normalized conversion efficiency .eta..sub.b can be increased by about 20%.
On the other hand, for efficient optical parametric oscillation utilizing the above-mentioned QPM, the distance between the reflecting mirrors of the oscillator, which are separately set at the both ends of an optical crystal, is changed to achieve harmonization with the excitation wave input as an oscillation source.
Further, a method using Q switching may be employed to obtain the oscillation output by such laser as pulsed output having a greater peak. The structure of such Q switching may be one wherein a saturatable absorption cell formed separately from optical crystal is placed on or off the oscillation path, or one wherein ultrasound output to be applied to an ultrasound switch formed separately from an optical crystal is changed, which are disclosed in Reference 16 and others.
According to the above-mentioned prior art techniques, almost all parts in the structures used to obtain output light by harmonic based on QPM or by frequency conversion, or optical parametric oscillation based on QPM, have individual structures. In these structures, a fine oscillation adjustment operation to achieve oscillation by changing the distance between the reflecting mirrors of the oscillator, fine adjustment operation to align optical axis with the direction of the optical axis of the lens and Q switch, and the like are necessary.
Therefore, an optical device comprising a single element constituting these parts, which permits electronic adjustment operation, will be extremely convenient for users. It remains to be solved, however, how to constitute such optical device.